This invention relates to controllable generators of digital pulse sequences, and more specifically to controllable digital frequency generators.
In the prior art there are numerous digital circuits which operate on pulse sequences, and which may require specified controlled variation of various aspects of the sequences. One example of such a circuit is a digital phase locked loop, which receives as an input a pulse sequence of a particular frequency and which may controllaby vary the phase of the input sequence in order to conform to the phase of a different sequence. In another example, it may be required to provide a controllably variable frequency for a pulse sequence.
Prior art generators of variable frequency pulse sequences are known, as illustrated by Jamieson U.S. Pat. No. 3,651,414, Berney U.S. Pat. No. 4,007,408 and Ransom U.S. Pat. No. 3,096,483.
However, such prior art variable generators of pulse sequences having variable characteristics require complex circuitry, utilizing programmable counters, for example, and suffer from a further disadvantage in the lack of simple circuitry capable of storing the control signals in order repeatedly to provide the desired variation of a pulse characteristic.
The above reference to a simple storage capability relates, for example, to an ability of a digital frequency generating circuit to respond on a continuing basis to a one time control input. That is, to the ability of the circuit to respond to a single control pulse or to a single sequence of control pulses by permanently varying the frequency of the generated output pulse sequence, or by repeating a phase shifting operation for each successive cycle of the output pulse sequence. However, such a capability is required in digital circuits operating as second order phase locked loops.
A major disadvantage of the prior art is the necessity for storage circuits for storing control input sequences, or for storing predetermined conditions, in order to provide a continuing (or substantially constant) frequency for the generated pulse sequence in response to a single application of the control input. Whether such storage is provided in the form of actual numeric storage or in the form of storage of a divisor for a variable counter or divider used in the circuitry, the resulting circuits are complex and thus expensive to fabricate.
There is accordingly a need in the prior art for a pulse sequence generating circuit having a controllable frequency with simplified storage capabilities for information provided in a one time application of a control sequence.